Method and apparatus for equalization and crosstalk mitigation

ABSTRACT

A method and apparatus for noise cancellation in a multi-channel communication system is disclosed. In one embodiment this system is configured to cancel FEXT on a victim channel utilizing the signals received on the other channels. The processing benefits gained by a receiver&#39;s other filters, such as for example, the FFE and DFE filters, is utilized when generating a FEXT cancellation signal. As a result, the complexity of the apparatus that generates the FEXT cancellation signal may be made less complex since part of the processing burden is performed by other filter apparatus. In one configuration pre-code FEXT cancellation occurs in that a pre-code FEXT filter generates one or more pre-code FEXT cancellation signals corresponding to each of the other channels. The pre-code FEXT cancellation signals are combined, prior to transmission, with the signals associated with each of the other channels, to thereby pre-cancel FEXT prior to transmission.

PRIORITY CLAIM

This application claims priority to provisional patent applicationhaving Ser. No. 60/424,961 filed on Nov. 7, 2002 having title METHOD ANDAPPARATUS FOR EQUALIZATION AND CROSSTALK MITIGATION and provisionalpatent application having Ser. No. 60/471,180 filed on May 16, 2003having title METHOD AND APPARATUS FOR EQUALIZATION AND CROSSTALKMITIGATION.

FIELD OF THE INVENTION

The invention relates to communication systems and, in particular, to amethod and apparatus for equalization and crosstalk mitigation.

RELATED ART

Modem communication systems achieve data communication betweentransceivers located at remote locations. To increase data communicationrates, communication system cabling arrangements often include numerousconductors in close proximity to transport a signal between remotelocations. These systems may be referred to as multi-channelcommunication systems. Furthermore, communication devices are oftenconstructed on circuit boards containing numerous conductors, traces, orelectrical devices. In all of these instances, coupling between channelsof a multi-channel communication system may occur, thereby introducinginterference into the other channels. This type of interference isgenerally referred to as crosstalk.

As is commonly understood, crosstalk may be characterized as near endcrosstalk (NEXT) and far end crosstalk (FEXT), depending on the sourceof the crosstalk that is introduced and the recipient or victim of thecrosstalk. FIG. 1 illustrates an example transceiver system with FEXTcoupling. As shown a first set of transceivers 104A-104D is part ofStation A 102, which is located at a first location. Station A 102communicates over channels 108A-108D with a second set of transceivers112A-112D that are part of Station B 110 and which are located at asecond location. FEXT type crosstalk is shown in FIG. 1 by couplinglines 116AB, 116AC, and 116AD with channel 108A as a victim channel.Signals on each of the adjacent channels, i.e., the disturber signals onthe disturber channels, often couple into the victim channel 108A andthereby interfere with reception of the desired signal being transmittedon the victim channel. For example, the signal on channel 108A will havecoupling from the signals transmitted onto Channels 108B-108D.

Similarly, the signal transmitted over the victim channel 108A maycouple onto the other channels 108B-108D. These coupling signals areshown in FIG. 1 as coupling signals 120BA, 120CA, and 120DA. Hence, theprocessing and decoding of the signals transmitted over channels108A-108D is made more difficult by the coupling that occurs betweenchannels.

While attempts have been made to overcome the effects of coupling, noneof these attempts adequately reduces the presence or effects ofcrosstalk. One such attempt is detailed in U.S. Pat. No. 6,236,645issued to Agazzi. The Agazzi reference teaches a cancellation systemassociated with each receiver in a multi-receiver system. Thecancellation system disclosed in the Agazzi reference may becharacterized as utilizing tentative decisions to reduce the effects ofcoupling onto a signal by making assumptions about the signal, such as asymbol value, that was sent on another channel. The tentative decisionmay be described as a guess regarding a symbol value that was sent onthe channel.

The Agazzi reference does not, however, eliminate sufficient coupling toovercome all the drawbacks of the prior art, and hence, even whenadopting the teachings of the Agazzi reference, coupling continues tointerfere with isolation of the received signal. One particular drawbackto the teachings of the Agazzi reference is that the system of theAgazzi reference continues to suffer from decision device errorresulting from crosstalk corruption of a signal because it makestentative decisions based on the analysis of a signal that includes anunacceptable amount of noise or coupling. Incorrect decisions may occuras a result, thereby increasing error rates. Further, the filterproposed for use by the Agazzi reference is undesirably complex, sinceit must span the convolution of the channel response with the couplingresponse. This undesirably limits processing speeds.

Furthermore, prior art solutions often do not address many aspects ofcoupling signal cancellation. Such aspects include coupling that occursat frequencies that differ from that of the disturber signal and signalsthat couple into the victim signal yet propagate through the victimchannel at rates different from that of the disturber signal.

The method and apparatus disclosed herein overcomes the drawbacks of theprior art and enables more accurate signal decoding and processing thanpreviously possible. Moreover, transmission at higher data rates withlower error rates, as compared to the prior art, is also enabled.

SUMMARY

To overcome the drawbacks of the prior art, disclosed herein is amulti-channel communication system having a first station and a secondstation configured to communicate over two or more channels. In oneembodiment, this system comprises a first station having two or moretransmitters configured to send two or more transmitted signals over twoor more channels from the first station to the second station. Also partof this embodiment is a second station having two or more receiversconfigured to process a received signal wherein each received signalcomprises the transmitted signal and one or more coupling signals. Inthis embodiment at least one of the receivers may comprise a decisiondevice configured to generate a decision output based on at least thereceived signal and a modified decision output. A feedback system isprovided and configured to generate the modified decision output andcombine the modified decision output, the received signal, and one ormore incoming FEXT cancellation signals. An adder is part of thereceiver and is configured to add the modified decision output from thedecision output to create an intermediate signal and one or more ELFEXTfilters are also provided and configured to process the intermediatesignal to create one or more outgoing FEXT cancellation signals.

It is contemplated that the communication system may be furtherconfigured to transmit data from the second station to the firststation. In addition, the one or more incoming cancellation signalscomprise one or more cancellation signals configured to remove FEXTcoupling from the received signal and the adder may be furtherconfigured to add the modified decision output to one or more incomingcancellation signals arriving from the decision output to create theintermediate signal. In one embodiment the ELFEXT filter is configuredto account for an ELFEXT portion of FEXT coupling. In one embodimenteach receiver at the second station generates a unique cancellationsignal tailored for each of the other receivers at the second station.It is contemplated that the feedback system may comprise a decisionfeedback filter and that the decision device may comprise a slicer. Inone embodiment, the multi-channel communication system comprises afour-channel communication system configured to operate in accordancewith an Ethernet Communication Standard and each transmitter maycomprise one or more FEXT precode filters configured to generate andprovide one or more precode cancellation signals to other transmitters.

Also disclosed herein is a multi-channel communication system configuredto reduce noise. In one embodiment this system comprises one or moretransmitters configured to transmit a first signal on a first channeland a second signal on a second channel. Also part of this system is afirst receiver configured to receive a third signal on the first channeland a second receiver configured to receive a fourth signal on thesecond channel, wherein the third signal comprises the first signal anda first interference component and the fourth signal comprises thesecond signal and a second interference component.

In this configuration, the first receiver comprises the following parts.A first feedback filter loop that is configured to receive the thirdsignal and reduce interference on the third signal such that the outputof the first feedback filter loop comprises a first feedback filter loopoutput and a first decision device having a first decision device outputconfigured as part of the first feedback filter loop. The first deviceis provided and configured to receive a second cancellation signal fromthe second receiver and combine the second cancellation signal with thefirst feedback filter loop output. A first filter is configured toreceive at least the decision device output and generate a firstcancellation signal.

A second receiver comprises a second feedback filter loop that isconfigured to receive the fourth signal and reduce interference on thefourth signal such that the output of the second feedback filter loopcomprises a second feedback filter loop output. A second decision deviceis provided to create a second decision device output configured as partof the second feedback filter loop. A second device is configured toreceive the first cancellation signal from the first receiver andcombine the first cancellation signal with the first feedback filterloop output. A second filter is configured to receive at least thesecond decision device output and generate the second cancellationsignal.

In one embodiment, the first device and the second may comprise summingjunctions. In one embodiment the first feedback filter loop and thesecond feedback filter loop both comprise a decision feedback filterconfigured to reduce intersymbol interference. In one configuration themulti-channel communication system has four channels and theinterference comprises FEXT coupling. The first filter and the secondfilter may comprise digital filters having coefficient values selectedto generate cancellation signals that cancel FEXT coupling. In oneembodiment, the one or more transmitters further comprise precode FEXTfilters such that each precode FEXT filter is configured to generate acancellation signal that can be combined with a signal, prior totransmission of the signal, to pre-cancel FEXT coupling. Moreover, atleast one of the one or more transmitters may be configured to generatean outgoing precode cancellation signal and receive an incoming precodecancellation signal from another transmitter.

In another embodiment a receiver for use in a multi-channelcommunication system to cancel coupling on a transmitted signal andreduce intersymbol interference is provided wherein a distorted versionof a transmitted signal and FEXT coupling comprise a combined signal.The receiver comprises a first device configured to receive and combinea feedback signal with the combined signal to create a decision deviceinput signal and a decision device configured to process the decisiondevice input signal to generate a discrete output. A decision feedbackequalizer is also provided and is configured to receive and process thediscrete output to generate an equalizer output. A second device isprovided to combine an incoming cancellation signal with the equalizeroutput to create the feedback signal. Also part of this embodiment areone or more ELFEXT filters, each configured to generate an outgoingcancellation signal that is related to the discrete output, wherein theoutgoing cancellation signal is tailored to cancel FEXT coupling onanother channels in the multi-channel communication device.

In one variation, the system further comprises a third device configuredto combine the discrete output and the one or more delayed cancellationsignals to create an input to the decision feedback equalizer. Thedecision device may comprise a slicer having ten output levels. In oneconfiguration, each multi-channel communication system comprises astation and each station comprises four receivers.

In at least one embodiment a receiver is provided for use in amulti-receiver system and is configured to receive two or more signalsvia two or more channels. In one configuration each respective receivercomprises an input configured to accept a received signal and a decisiondevice configured to quantize a decision device input signal to one oftwo or more decision values, such that the decision device input signalis based on the received signal. A first filter is also part of thereceiver and is configured to process the decision values to create afirst filtered signal. One or more second filters are configured toprocess the decision values and the first filtered signal to create anoutgoing cancellation signal tailored to cancel coupling on one or moreother channels. Also part of this embodiment are one or more additionaldevices configured to receive one or more incoming cancellation signalsfrom other receivers in the multi-receiver system and process the one ormore incoming cancellation signals, the first filtered signal, and thereceived signal to cancel unwanted coupling in the received signal.

In one embodiment, the first filter comprises a digital filterconfigured to generate a feedback signal that reduces intersymbolinterference. In one embodiment the decision device quantizes thedecision device input signal to any one of ten values based on acomparison to predetermined thresholds. It is further contemplated thatthe one or more second filters comprise digital filters having two ormore coefficients and the one or more second filters and the firstfilter are configured to cancel coupling and reduce intersymbolinterference. The receiver may further comprise a third filtercomprising a feed forward filter that is configured to process thereceived signal to reduce intersymbol interference.

Also disclosed is a method for reducing interference in a multi-channelcommunication system having two or more receivers and two or morechannels. In one embodiment the method comprises receiving a firstsignal on a first channel with a first receiver and a second signal on asecond channel with a second receiver and then combining a feedbacksignal with the first received signal to create a first combined signal.Next, processing the first combined signal to reduce intersymbolinterference in the first combined signal to create a processed signalwherein the interference was created by passage of the first signalthrough the first channel. Next, the method combines the processedsignal with at least a first cancellation signal received from at leastthe second receiver to create a feedback signal. Thereafter, the methodcombines the feedback signal with the first combined signal to create asecond combined signal and then processes the second combined signal togenerate at least a second cancellation signal.

In one embodiment the step of combining a feedback signal with the firstreceived signal cancels FEXT coupling in the first received signal. Inone embodiment, the step of processing the first combined signalcomprises performing decision feedback equalization on the signal togenerate a signal that reduces intersymbol interference. Processing thesecond combined signal to generate at least a second cancellation signalmay comprise filtering the second combined to isolate ELFEXT coupling.In one configuration, the second receiver is configured similarly to thefirst receiver and the second receiver generates the first cancellationsignal and receives the second cancellation signal from the firstreceiver. In addition, the method may further comprise delaying thefirst cancellation signal to achieve proper timing.

A receiver configuration is also disclosed for FEXT cancellation in amulti-channel communication system comprising a feedback loop thatcomprises a first device configured to combine a received signal with afeedback signal and one or more incoming cancellation signals to createa combined signal. Part of this embodiment is also a decision deviceconfigured to process the combined signal to generate a decision outputand a first filter. The first filter is configured to generate thefeedback signal based on the decision output and the one or moreincoming cancellation signals or a delayed version of the one or moreincoming cancellation signals, wherein the one or more incomingcancellation signals are received from one or more other receivers inthe multi-channel communication system. Also part of this embodiment isone or more second filters configured to receive at least the decisionoutput and generate one or more outgoing cancellation signals that arethen routed to other receivers in the multi-channel communicationsystem.

In one embodiment, the first device comprises a subtractor or summingjunction, the first type filter is configured to account for the effectsof the channel, and the one or more second type filters are configuredto account for coupling. In one embodiment FEXT cancellation isperformed by the first filter and an incoming cancellation signal. Inone embodiment, the receiver further comprises a feed-forward filterconfigured to process the received signal prior to the received signalarriving at the feedback loop. In one configuration, the receiver isassociated with each channel in a four-channel communication system andeach receiver receives an incoming cancellation signal from each of theother receivers.

Also disclosed herein is a method for canceling coupling in amulti-channel communication system having two or more receiverscomprising the steps of receiving a signal over a channel and alsoreceiving at least one cancellation signal from at least one of theother receivers in the multi-channel communication system. The methodthen processes the signal to account for the effect of the signalpassing through the channel thereby generating a processed signal andcombining the processed signal and the one or more cancellation signalsfrom the other receivers to generate a feedback signal. Thereafter,combining the feedback signal with the received signal to cancelcoupling in the received signal and then generating one or more outgoingcancellation signals. The method then provides at least one outgoingcancellation signal to at least one of the other receivers in themulti-channel communication system.

This method may also include generating one or more outgoingcancellation signals by generating one or more cancellation signals witha filter configured to isolate the ELFEXT coupling. In one embodiment,the method further comprises combining the feedback signal with the oneor more cancellation signals from the other receivers prior to combiningthe feedback signal with the received signal to reduce noise in thereceived signal. The processing may further comprise quantizing thecombination of the received signal and the one or more cancellationsignals to one of one or more discrete levels prior to processing.

Also disclosed herein is a method and apparatus to perform processing atthe transmitter to thereby pre-cancel or precode unwanted coupling thatwill couple onto the transmitted signal during passage of the signalthrough the channel. In one embodiment a transmit system is configuredas part of a first station such that the first station has two or moretransmitters each of which are associated with a channel. Thetransmitters are configured as part of a multi-channel communicationsystem that is configured to modify a data signal prior to transmissionby the transmitter from the first station to a second station to reducethe effects of coupling on the data signal. In one embodiment at leastone of the transmitters in the transmit system comprises an inputconfigured to receive a data signal, the data signal to be transmittedover a first channel after processing by the transmitter. Also part ofthis system is one or more filters configured to generate one or moreoutgoing cancellation signals, the one or more outgoing cancellationsignals to be provided to one or more other transmitters in the transmitsystem to cancel, prior to transmission of the data signal, FEXTcoupling from the first channel to one or more other channels. A deviceis also provided as part of this embodiment that is configured tocombine one or more incoming cancellation signals from the one or moreother transmitters within the transmit system with the signal so thatthe one or more incoming cancellation signals arriving from the one ormore other transmitters in the multi-channel communication system areconfigured to cancel, prior to transmission of the data signal, FEXTcoupling, that will couple, from the one or more other channels to thefirst channel.

In one embodiment, the device comprises a subtractor configured tosubtract the one or more incoming cancellation signals from the datasignal. In one embodiment, the one or more filters comprise digitalprecode FEXT filters. For example in one embodiment the digital precodeFEXT filter is associated with each of the other transmitters in thetransmit system and each digital precode FEXT filter is configured togenerate an incoming cancellation signal. It is also contemplated thatthe transmitter may further comprise a transmit precode filter inaddition to the one or more filters configured to generate one or moreoutgoing cancellation signals.

Also disclosed is a coupling precode filter system configured to modifytwo or more signals in a multi-transmitter, multi-channel transmitsystem to cancel, prior to transmission, FEXT coupling that may occurduring transmission of the two or more signals through the two or morechannels. In one embodiment, this system comprises a first inputconfigured to receive a first signal and a second input configured toreceive a second signal. A first filter is also provided and isconfigured to process the first signal to generate a first cancellationsignal so that the first cancellation signal cancels at least a portionof the coupling that will couple from the first signal onto the secondsignal during transmission. A second filter is also provided and isconfigured to process the second signal to generate a secondcancellation signal so that the second cancellation signal cancels atleast a portion of the coupling that will couple from the second signalonto the first signal during transmission. Also part of this system is afirst device, configured to combine the second cancellation signal withthe first signal prior to transmission of the first signal, and a seconddevice configured to combine the first cancellation signal with thesecond signal prior to transmission of the second signal.

In one variation of this embodiment the first filter and second filterare configured as non-causal filters. In one embodiment the first filteris located in a first transmitter and the second filter is located in asecond transmitter and each of the first transmitter and the secondtransmitter further comprise a transmit precode filter. It iscontemplated that the coupling precode filter system may be configuredto operate in a four-channel environment and thereby further comprises athird filter and a fourth filter.

Also disclosed herein is a method, for use in a multi-channelcommunication system having two or more transmitters, for FEXTcancellation of coupling from a first signal transmitted on a firstchannel to a second signal transmitted on a second channel. Such amethod may comprise receiving a first signal at a first transmitter andthen performing first processing on the first signal to create a firstprocessed signal. The method also routes the first processed signal toone or more first transmitter cancellation filters and performs secondprocessing on the first processed signal with the one or more firsttransmitter cancellation filters to create a cancellation signal.Thereafter, routing the cancellation signal to a second transmitter andcombining, prior to transmission, the cancellation signal with a secondsignal being processed by a second transmitter to reduce the effects ofcoupling of the first signal onto the second signal during transmission.

In this configuration, the precode FEXT filter is configured as anon-causal filter. In addition, the multi-channel communication systemmay comprise at least one station having four transmitters, each ofwhich is associated with a channel. Moreover, the method may furthercomprise receiving a second cancellation signal at the first transmitterand combining the second cancellation signal with the first processedsignal to reduce the effects of coupling from the second signal onto thefirst processed signal during transmission. For example, the combiningmay comprise subtracting the cancellation signal from the second signal.

Also disclosed is a method of FEXT cancellation in a four-channelcommunication system, wherein a transmitter is associated with each ofthe first channel, second channel, third channel, and fourth channel. Inone embodiment the method may comprise receiving a first signal, secondsignal, third signal, and fourth signal at each of a first transmitter,second transmitter, third transmitter, and fourth transmitterrespectively and processing the first signal to generate a secondtransmitter cancellation signal, a third transmitter cancellationsignal, and a fourth transmitter cancellation signal. The method alsocomprises routing the second transmitter cancellation signal, the thirdtransmitter cancellation signal, and a fourth transmitter cancellationsignal to the second transmitter, third transmitter, and fourthtransmitter respectively and then combining the second transmittercancellation signal, the third transmitter cancellation signal, and thefourth transmitter cancellation signal with the second signal, thirdsignal, and fourth signal respectively. The step of combining cancelsthe effects of FEXT coupling onto the second channel, third channel, andfourth channel that will occur during transmission of a signal on thefirst channel.

In one embodiment of this method the step of processing the first signalcomprises routing the first signal to a first precode FEXT filter, asecond precode FEXT filter, and third precode FEXT filter and thenprocessing the first signal in each of the precode FEXT filters tocreate the second transmitter cancellation signal, the third transmittercancellation signal, and the fourth transmitter cancellation signal.This method may further comprise receiving at the first transmitter oneor more incoming cancellations signals from each of the second, thirdand fourth transmitters in the four channel communication system andthen combining the one or more incoming cancellations signals from eachof the other transmitters with the first signal to cancel the effects ofFEXT coupling on the first signal. In one variation the method furthercomprises performing transmit precode filtering on the first signal andthe processing is performed by one or more digital filters. It iscontemplated that the transfer function of the one or more digitalfilters may be selected to cancel ELFEXT.

Other systems, methods, features and advantages of the invention will beor will become apparent to one with skill in the art upon examination ofthe following figures and detailed description. It is intended that allsuch additional systems, methods, features and advantages be includedwithin this description, be within the scope of the invention, and beprotected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The components in the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.In the figures, like reference numerals designate corresponding partsthroughout the different views.

FIG. 1 illustrates a block diagram of an example embodiment of a twostation communication system.

FIG. 2 illustrates a block diagram of a receiver/transmitter pair.

FIG. 3 illustrates a block diagram of an example embodiment of amulti-channel point-to-point communication system.

FIG. 4 illustrates a block diagram of an example embodiment of atransmitter.

FIG. 5 illustrates a block diagram of a FEXT model wherein FEXT thatcouples from a disturber channel onto a victim channel passes throughthe victim channel.

FIG. 6 illustrates a block diagram of an example embodiment of areceiver configured based on the invention described herein.

FIG. 7 illustrates a block diagram of a FEXT model wherein FEXT thatcouples from a disturber channel onto a victim channel passes throughthe disturber channel prior to coupling.

FIG. 8 illustrates a block diagram of an alternative embodiment of areceiver configured according to the present invention.

FIG. 9 illustrates a block diagram of an example embodiment of areceiver configured in a multi-channel configuration.

FIGS. 10A-10C illustrate a block diagram of an example embodiment of atransmitter configured in a multi-channel configuration.

FIGS. 11A-11C illustrate a block diagram of an alternative exampleembodiment of a transmitter configured in a multi-channel configuration.

FIGS. 12A and 12B illustrate an operational flow diagram of a generalexample method of coupling cancellation.

FIG. 13 illustrates an operational flow diagram of particular examplemethod of coupling cancellation.

DETAILED DESCRIPTION

In reference to FIG. 2, a block diagram of a receiver/transmitter pairis shown. A channel 212 connects a first transceiver 230 to a secondtransceiver 234. The first transceiver 230 connects to the channel 212via an interface 244. The interface 244 is configured to isolate theincoming and outgoing signals. The channel 212 may comprise more thanone conductor, and hence the interface 244 may perform isolation foreach channel based on direction of data flow. The receive module 238 andtransmit module 242 may comprise any assembly of hardware, software, orboth configured to operate in accordance with the principles describedherein.

The receive module 238 and transmit module 242 communicate with aprocessor 246. The processor 246 may include or communicate with amemory 250. The processor operates as described below in more detail andas would be understood by one of ordinary skill in the art. The memory250 may comprise one or more of the following types of memory: RAM, ROM,hard disk drive, flash memory, or EPROM. The processor 246 may beconfigured to perform one or more calculations or signal analysis. Inone embodiment, the processor 246 is configured to execute machinereadable code stored on the memory 250. The processor 246 may performadditional signal processing tasks as described below.

The second transceiver 234 is configured similarly to the firsttransceiver 230. The second transceiver 234 comprises an interface 252connected to a receiver module 256 and a transmitter module 260. Thereceiver module 256 and a transmitter module 260 communicate with aprocessor 264, which in turn connects to a memory 268. Operation occursas described below in more detail.

FIG. 3 illustrates a block diagram of an exemplary multi-channelpoint-to-point communication system. One exemplary application of such amulti-channel communication system is a multi-gigabit transceiverutilizing any category or class of unshielded twisted pair (UTP) cablesupporting Ethernet protocols. As shown, it includes a physical codingsublayer (PCS) 302, 304 shown as coupled together over a channel 312. Inone embodiment, each channel comprises twisted pair conductors. Each ofthe channels 312 is coupled between transceiver blocks 320 through aline interface 306, and each channel is configured to communicateinformation between transmitter/receiver circuits (transceivers) and thephysical coding sublayer (PCS) blocks 302, 304. Although shown with fourchannels for purposes of discussion, any number of channels andassociated circuitry may be provided. In one embodiment, thetransceivers 320 are capable of full-duplex bi-directional operation. Inone embodiment, the transceivers 320 operate at an effective rate ofabout 2.5 Gigabits per second.

FIG. 4 illustrates a block diagram of an example embodiment of atransmitter. This is but one exemplary embodiment of a transmitter. Itis contemplated that other configurations may be embodied by one ofordinary skill in the art. In the exemplary configuration of FIG. 4, adata source 400 connects to a mapping module 404, which in turn connectsto a transmit precode filter 408. It is contemplated that the transmitprecode filter 408 does not deal with FEXT cancellation. FEXTcancellation using a FEXT precode filter (not shown in FIG. 4) that isin addition to or supplemental to the transmit precode filter 408 isdiscussed below in more detail.

The data source 400 may comprise any source of data to be transmittedover a channel. In one embodiment, the data source 400 comprises aprocessing or networking layer of a communication protocol. In oneembodiment, the data source 400 comprises a network processing device,for example, a media access controller (MAC). In one embodiment, thedata arrives from application software executing on a computer.

The mapping module 404 comprises hardware, software, or a combination ofboth configured to transform the received binary data into one or moresymbols capable of representing one or more bits of binary data. Oneexample mapping that may occur is pulse amplitude modulation (PAM),wherein several bits of binary data are mapped into a single symbol.Another example mapping comprises quadrature amplitude modulation (QAM).Any type mapping may be utilized. Through mapping, transmission of asingle symbol achieves transmission of several bits of informationthereby increasing data transfer rates.

In addition to mapping, the mapping module 404 may incorporate forwarderror correction (FEC) coding. Examples of FEC coding compriseconvolutional coding and trellis coding. It is contemplated that themethod and apparatus described herein may be utilized with any form oferror correction, or without error correction.

The transmit precode filter 408, which is discussed below in greaterdetail, connects to the output of the mapping module 404 and comprises asignal modification device configured to manipulate the signal tocounter the distorting effects of the channel. The transmit precodefilter 408 may be configured as a digital filter having coefficientvalues set to achieve a desired level of signal modification. In oneembodiment, the transmit precode filter 408 comprises a finite impulseresponse filter adapted to at least partially negate the distortingeffects of a channel.

The output of the transmit precode filter 408 connects to a digital toanalog (D/A) converter 412 to transform the mapped signal to an analogformat suitable for transmission through a channel. Thereafter, thesignal is provided to a line driver/amplifier 416. The linedriver/amplifier 416 manipulates the signal to a power level suitablefor transmission over the channel. The degree or level of amplificationmay be dependant upon the power limits or specification as defined by aparticular communication protocol, crosstalk and coupling concerns, andthe distance to a receiver or a repeater. The output of the linedriver/amplifier 416 connects to a transformer/hybrid 420. Thetransformer/hybrid 420 provides isolation between transmit and receivesignals as well as the channel itself. The output of thetransformer/hybrid 420 connects to a channel.

Prior to discussion of further embodiments, additional discussion of farend crosstalk (FEXT) is warranted. In general, FEXT is defined as farend cross talk, and as such it is composed of, i.e., is the convolutionof, the channel response and the ELFEXT response. ELFEXT comprises theequivalent coupling at the far end of the channel measured with respectto an attenuated transmit signal. It is contemplated that the channelresponse can be that of the disturber channel or the victim channel.Removing FEXT is made difficult because FEXT is dependant on the lengthof the channel and since the FEXT signal may couple at the far end ofthe channel, the near end of the channel or anywhere in between. As aresult, the FEXT is also subject to ISI and attenuation as it passesthrough the victim channel. Considering FEXT as ELFEXT takes intoconsideration the effects of the channel, including the length andeffects of the channel. Prior art solutions did not adequately addresssuch aspects. Through consideration of this and other aspects, a morecomplete coupling cancellation system and method may be realized.

FIG. 5 illustrates a block diagram of an equivalent model of couplingoccurring prior to passage of the coupling through the channel. This isbut one possible system model. It is contemplated that other systemmodel configurations may be utilized without departing from the scope ofthe invention. As shown, a channel A transmitter 504 connects to achannel A 508 having a transfer function or impulse response shown byQ_(A)(z) in block 512. Channel A 508 connects to a channel B receiver516.

Similarly, a channel B transmitter 524 connects to a channel B 528having a transfer function shown by Q_(B)(z) in block 532. Channel B 528connects to a channel B receiver 536. Also included in FIG. 5 is thecoupling effect between channels shown within dashed line 540. Thesignal on channel B 528 couples into channel A 508. The transferfunction of the coupling is defined by block 544 as B_(A)(z). Likewise,block 548 defines the coupling from channel A into channel B asA_(B)(z).

As can be seen, the coupling in this example embodiment is shown asoccurring before the effects of the channel 512, 532. Thus, the FEXTsignal that couples from channel A to channel B is considered to beaffected by the transfer function of the channel Q_(B)(z) as it passesthrough channel B. Thus, the transfer function for the FEXT couplingfrom channel A to channel B can be described as the ELFEXT componentA_(B)(z) convolved with the transfer function of channel B Q_(B)(z).

FIG. 6 illustrates a block diagram of an example embodiment of a FEXTcancellation system based on a model of FEXT coupling prior to passagethrough the victim channel as shown in FIG. 5. In general, the system ofFIG. 6 operates to cancel FEXT on a victim channel utilizing the signalsreceived on the other channels. As an advantage of the embodiment shownin FIG. 6, the processing benefits gained by a receiver's other filters,such as for example, the FFE and DFE filters, are utilized whengenerating the FEXT cancellation signal. As a result, the complexity ofthe apparatus that generates the FEXT cancellation signal may be madeless complex since part of the processing burden is performed by otherfilter apparatus.

Turning now to FIG. 6, a channel A 604 connects to a feed forward filter(FFE) 608 having transfer function F_(A)(z). In one embodiment, the FFE608 is configured to reduce intersymbol interference. It is contemplatedthat one of ordinary skill in the art is capable of FFE 608 constructionand is familiar with basic FFE operation. Accordingly, the basicprinciples of FFE operation are not discussed in detail herein beyondthat associated with the new and distinctive features of the invention.It is further contemplated that filter or equalizer structures otherthan an FFE 608 may be utilized without departing from the scope of theinvention.

The output of the FFE 608 connects to a subtractor 612, and the outputof the subtractor connects to a decision device 616. The decision device616 is a device that quanitizes an input to one of two or more possiblevalues based on analysis of the input to one or more threshold values.In one embodiment, the decision device 616 operates in conjunction withPAM10 mapping to quantize the input s(n) to one of ten values. In oneembodiment, the decision device 616 analyzes the received signal'svoltage level, after processing, to determine the symbol sent over thechannel. The output of the decision device 616 may comprise any numberof discrete levels. As shown, the output of the decision device 616 isfed back into an adder 620 and into an ELFEXT filter 632. The output ofthe adder 620 is provided to a feedback filter 636 (DFE) having atransfer function of Q_(A)(z)−1. It is contemplated that one of ordinaryskill in the art is capable of DFE 636 construction and is familiar withbasic DFE operation. Accordingly, the basic principles of DFE operationare not discussed in detail herein beyond that associated with the newand distinctive features of the invention. It is further contemplatedthat filter or equalizer structures other than an FFE 608 or DFE 636 maybe utilized without departing from the scope of the invention. Althoughnot shown, it is also contemplated that one or more delays may beutilized as necessary and as would be understood by one of ordinaryskill in the art.

The terms adder, subtractor, summing junction or the like should beinterpreted broadly to mean any device configured to combine signals. Ascan be appreciated by one of ordinary skill in the art, subtraction isthe equivalent of adding a negative value.

The FFE 608 and the DFE 636 perform equalization on the received signalto compensate for the distorting effects of the channel. The DFE 636, aspart of the feedback, receives and weights past values, which aresubsequently added in the summing junction 640, then subtracted inelement 612. It is contemplated that the FFE 608 and DFE 636 may possesscoefficients, or other scaling values, associated with one or more tapsor stages of the FFE and the DFE. The coefficient values are selected toachieve desired signal equalization to thereby negate, reverse, orreduce the effects of the channel. In one embodiment, the FFE 608 andDFE 636 coefficient values are selected based on the principlesdescribed herein. In one embodiment, the coefficient values are arrivedat using a least mean squared algorithm. In one embodiment, thecoefficient values of the FFE 608 and DFE 636 are calculated andselected to counter the signal distorting effects of the channel whileminimizing noise amplification and minimizing the undesirable effects oferror propagation through the DFE feedback loop.

The ELFEXT filter 632 comprises a filter configured to estimate theELFEXT transfer function to thereby account for the ELFEXT. Statedanother way, the ELFEXT filter 632 has a transfer function A_(B)(z) thatmirrors the transfer function of the coupling from one channel toanother. The ELFEXT filter 632 may comprise any processing system ordevice configured to generate a cancellation signal. In this embodimentand as suggested by the model in FIG. 5, the coupling function of theELFEXT filter is prior to passage of the coupling through the channel.

The output of the adder 620 is provided to a feedback filter 636 shownas having a transfer function of Q_(A)(z)−1. In one embodiment, thefeedback filter 636 is configured to be an estimate of the transferfunction of the channel. The feedback filter output is provided to anadder 640 which combines the feedback filter output with a cancellationsignal from one or more other channels. In this embodiment, thecancellation signal is received from a channel B cancellation system,which is discussed below in more detail. The output of the adder 640 isprovided to subtractor 612, where the cancellation signal is subtractedfrom the received signal. Since the cancellation signal is subtractedprior to the decision device 616, the decision by the decision devicehas a higher likelihood of being correct. This improves systemperformance and accuracy.

The apparatus of channel B is now discussed. Channel B 650 connects to afeed forward filter 654, the output of which connects to a subtractor658. The output of the subtractor 658 feeds into a decision device 662.The decision device has an output that feeds into an adder 666 and anELFEXT filter 670. The ELFEXT filter 670 has a transfer functionB_(A)(z) and an output that connects to the adder 640, which isdiscussed above, and into a delay 674. The summing junction 666 alsoreceives an input from a delay 678. The delay 678 receives an input fromthe ELFEXT filter 632. The delays 674, 678 account for propagation delaydifferences in Q_(A)(z) and Q_(B)(z) and/or to account for processingdelay.

The output of summing junction 666 connects to a feedback filter 680which has a transfer function defined as Q_(B)(z)−1. The output of thefeedback filter 680 is provided to an adder 684, which also receives asan input the output of the ELFEXT filter 632. The adder 684 produces thecancellation signal which is subtracted from the signal received onchannel B in subtractor 658.

The following discussion details operation of the cancellation of thesignal received on a victim channel, in this example discussion channelA, from a disturber channel, in this example discussion channel B. It iscontemplated that the principles shown and discussed herein may beextended to any number of channels. The system shown in FIG. 6 receivesa signal on channel A 604 and on channel B 650. Both signals areprocessed in a generally similar manner by the FFE 608, 654, thedecision device 616, 662, and the feedback filter 636, 680. Ofimportance, the output of decision device 662 is provided to the ELFEXTfilter 670 for processing. The output of decision device 662 is assumedto be an accurate decision of the actual signal sent from transmitter B.The ELFEXT filter 670 is configured or trained to have a transferfunction B_(A)(z) representing the coupling from channel B to channel A,but it does not represent the effect or transfer function of channel A.The output of the ELFEXT filter is provided to the adder 640, wherein itis added with the output of the feedback filter 636. As stated above,the feedback filter is configured or trained to have a transfer functionof that of the channel with which it is associated. As can be seenvisually in FIG. 6, the cascading of the ELFEXT filter output and thefeedback filter output results in a cancellation signal that accountsfor the FEXT coupling onto channel A from channel B and for the passageof this FEXT coupling through channel A. The delays are provided toaccount for different propagation delay characteristics in Q_(A)(z) andQ_(B)(z) or a signal processing path associated therewith.

As an advantage to this embodiment, the processing complexity of thisoperation is distributed between the filters 636, 670 for channel A andfilters 680, 632 for channel B. As a result, the complexity of eachfilter is reduced. Furthermore, the feedback filter is already acomponent in many signal processing systems, and by utilizing itsalready available output, the process of FEXT cancellation may berealized with minimal additional processing requirements. If a singlefilter is tasked with generating the FEXT cancellation signal, such asingle filter would be undesirably complex.

FIG. 7 illustrates a block diagram of an equivalent model of FEXTcoupling occurring after passage of the disturber signal through thedisturber channel. As shown, a channel A transmitter 704 connects to achannel A 708 having a transfer function or impulse response shown byQ_(A)(z) in block 712. Channel A 708 connects to a channel A receiver716.

Similarly, a channel B transmitter 724 connects to a channel B 728having a transfer function or impulse response shown by Q_(B)(z) inblock 732. Channel B 728 connects to a channel B receiver 736. Alsoincluded in FIG. 7 is the coupling effect between channels shown withindashed line 740. The signal on channel B 728 couples into channel A 708.The impulse response or transfer function of the coupling is defined byblock 744 as B_(A)(z). Likewise, block 748 defines the coupling fromchannel A into channel B as A_(B)(z).

As can be seen, the coupling in this example embodiment is shown asoccurring after the effects of the channel 708, 728. Thus, the FEXTsignal that couples from channel A to channel B is considered to beaffected by the transfer function of the channel Q_(A)(z) as it passesthrough channel A. Thus, the transfer function for the FEXT couplingfrom channel A to channel B can be described as the transfer function ofthe ELFEXT A_(B)(z) convolved with the transfer function of channel BQ_(A)(z). This is in contrast to the model shown in FIG. 5, wherein theELFEXT coupling from channel A onto channel B is subject to thedistorting effects of passage through channel B instead of channel A.

FIG. 8 illustrates a block diagram of an example embodiment of a FEXTcancellation system based on a model of FEXT coupling after passagethrough the disturber channel. In general, this block diagram is basedon the model of FIG. 7, and as such, it is configured to cancel FEXTcoupling from a disturber channel to a victim channel with theassumption that the coupling signal first passes through disturberchannel prior to coupling onto the victim channel.

As shown in FIG. 8, a channel A 804 connects to a feed forward filter808, the output of which connects to a subtractor 812. A decision device816, such as a slicer, receives the output of the subtractor 812 andprovides its output to a feedback filter 820 and an adder 824. The adder824 also receives an input of the feedback filter 820 output. The outputof the adder 824 is provided to an ELFEXT filter 828 having a transferfunction of A_(B)(z). The ELFEXT filter 828 is configured or trained tohave a transfer function A_(B)(z) based on the ELFEXT that couples fromchannel A to channel B. Accordingly, the output of the ELFEXT filter 828is a cancellation signal that is provided to channel B cancellationsystem which is now described.

Channel B connects to a FFE 858, which in turn connects to a subtractor862. The output of the subtractor 862 feeds into a decision device 864,which has an output that connects to both of a feedback filter 866 andan adder 870. The adder 870 also receives as an input the output fromthe feedback filter 866, as does an adder 874. The output of the adder870 feeds into an ELFEXT filter 880 having a transfer function definedas B_(A)(z). The output of this ELFEXT filter 880 is connected to anadder 832 that is associated with channel A. In turn, the output ofadder 832 connects to subtractor 812. Similarly, the output of ELFEXTfilter 828 is connected to adder 874, which in turn connects tosubtractor 862.

In operation, it is contemplated that the signal received over channelA, the victim channel in this example discussion, may contain FEXTcoupling from the disturber channel, in this example, channel B. Tocancel this FEXT coupling and thereby improve the accuracy of thedecision device 816, the signal is received on channel B 854 andprocessed by the FFE 858 and the feedback filter 866 to create thesignal at conductor 890. Thereafter, this signal is added to theaccurate decision from the decision device 864 by the adder 870 tocreate the input to the ELFEXT filter 880. To this point, the processingthat has occurred on the signal received over channel B 854 accounts forthe effects of channel B. To account for the effects of the ELFEXT,i.e., the amount of coupling and not the effects of the channel, theELFEXT filter 880 is configured or trained to have a transfer functionthat will create a signal that approximates the amount of coupling fromchannel B to channel A. The output of the ELFEXT filter 880 thuscomprises the cancellation signal that is provided to the channel Areceiver via the adder 832, then to the subtractor 812 to be removedfrom the received signal. In this manner, FEXT coupling may be removed.

As stated above in conjunction with FIG. 6, as an advantage to thisembodiment, the processing complexity of this operation for FEXTreduction in the victim channel is distributed between the filters 866,880. As a result, the total complexity is reduced. Furthermore, thefeedback filter 866 is already a component in many signal processingsystems, and by utilizing its already available output, the process ofFEXT cancellation may be realized with minimal additional processingrequirements, namely filter 880. If a separate single filter is taskedwith generating the FEXT cancellation signal, then such a single filterwould be undesirably complex.

It should be noted that the principle described above in conjunctionwith FIGS. 5-8 may be extended to more than two channels. FIG. 9illustrates a block diagram of an example embodiment of the receivershown in FIG. 8 in a multi-channel configuration, such as channel Athrough channel M, where the variable M comprises any positive integer.As compared to channel A shown in FIG. 8, similar elements are labeledwith similar reference numerals. As shown, input channels 804A-804M,where M is any positive integer, are received over a multi-conductorcommunication system having M number of channels. In one embodiment,there are more conductors than channels. The channels may comprise anymedium capable of carrying a signal or data, such as but not limited to,any category or class of copper cabling, wireless channels, fiber opticchannels or cables, free-space optic channels, twisted pair conductorsor any other conductive path, coaxial cables or other channels that arecurrently or that may become available in the future. Although shownwith four channels, i.e., where M equals four, it is contemplated thatthe principles described herein may be expanded to any number ofchannels or conductors.

Operation of elements 808, 816, and 820 occur as described above inconjunction with FIG. 8. Additional filters 828 are included as shown toaccount for the multi-channel configuration. Although not shown withconnecting lines, the output of each filter 828 is routed as an input toan adder junction 832. For example, the output of filter 828AB comprisesa cancellation signal A′_(B) that comprises a cancellation signal fromdisturber channel A that is provided to victim channel B. Accordingly,adder 832B receives the cancellation signal A′_(B) via input 900B andutilizes the cancellation signal A′_(B) to remove FEXT that has coupledfrom channel A onto Channel B. This process occurs for each of thefilters 828 and channels as shown.

It is contemplated that the filter coefficients in any of the filtersdiscussed in FIGS. 5-9 and in particular, the coefficients of ELFEXTfilter 828 in FIG. 9, may be established in any manner known in the art.Thus, the coefficients may be established during an initial trainingperiod or set at a default value during manufacture. In one embodiment,the least mean square algorithm is utilized to train or adapt the ELFEXTfilters. It is further contemplated that the filter coefficients may beupdated during system operation to thereby adapt to changing channel orenvironmental conditions. In one embodiment, training of the ELFEXTfilter occurs while the filter is separate from the channel, such aswhen the channel is not transmitting data and the effects of ELFEXT maybe isolated.

In some instances, the signals required to generate the FEXTcancellation signal do not arrive at a receiver concurrent with or priorto the arrival of the FEXT signals that are coupled onto the victimsignal. For example, the signals that are required to generate the FEXTcancellation signal are the signals that are transmitted on thedisturber channels, i.e., the signals on channels other than the victimchannel. Hence, if a signal on a disturber channel has not arrived atits associated receiver, then that signal is not available forprocessing to generate a cancellation signal. This is particularlytroubling when the signal coupling from the disturber channel onto thevictim channel has already arrived at the receiver associated with thevictim channel. Such differences in arrival times may occur because FEXTcoupling may propagate through the victim channel more quickly than itpropagates through the disturber channel. In addition, some channels ina multi-channel communication system are different lengths, therebycausing the signals to arrive at different times relative to a commontransmit time.

As a result, some FEXT coupling may be present as a component of thesignal on the victim channel even though the signal that generated thecoupling, i.e., the signal on the disturber channel, has not yet arrivedat the receiver associated with the disturber channel. This portion ofthe FEXT coupling may be referred to as the non-causal portion of theFEXT. In contrast, the causal portion of the FEXT may be defined as theFEXT components that arrive with or after the arrival of the signal thatgenerated the FEXT on the disturber channel.

Failure to account for the different arrival rate of FEXT components,i.e., the non-casual FEXT components, may hinder operation of the FEXTcancellation system. This is especially true as data communication ratesincrease, since timing becomes more critical and each processing stepshould be completed within constrained time limits. In one embodiment,the method and apparatus disclosed herein overcomes this challengeassociated with FEXT cancellation by incorporating FEXT cancellationoperations in the transmitter side of the communication system.

While it is contemplated that numerous filtering or FEXT cancellationsystems may be incorporated into the transmitter, in one embodiment aFEXT precode filter is tailored to perform FEXT cancellation in thetransmitter. This operation may also be referred to as pre-transmissionFEXT cancellation. The term precode filter as used herein is defined tomean a filter located in the transmitter. In addition, the concept ofFEXT precoding occurs in addition to a possible transmit precode filterthat is identified as element 408 in FIG. 4.

In general, one or more FEXT precode filters may be located in one ormore of the transmitters in a multi-channel communication system and maybe trained to have a transfer function that will modify a signal tothereby counter, in advance of transmission, FEXT coupling that willoccur on the signal passing through the victim channel or that willcouple to other channels. Use of a FEXT precode filter allows thetransmitter to counter a portion or all of the FEXT coupling prior totransmission. In one embodiment, the non-causal aspects of the FEXTcoupling, that is, the FEXT coupling that arrives on the victim channelafter the arrival of that portion of the signal that generated thecoupling, is pre-cancelled in the transmitter.

In one embodiment, precode FEXT cancellation comprises measuring theFEXT response for a channel at the receiver and dividing by the impulseresponse of the line to obtain the FEXT precode filter coefficients. Inanother embodiment, the FEXT precode filter is trained using referencebased training. In yet another embodiment, the filter coefficients arederived by training the filters associated with only one channel at atime.

In one embodiment, all or a portion of the FEXT cancellation isperformed by a FEXT precode filter such that the FEXT precode filterisolates the FEXT transfer function for each of the other channels in amulti-channel communication system and provides its output to each ofthe other transmitters. The precode FEXT cancellation signal, which isgenerated by the FEXT precode filter, is combined with the signals beingtransmitted on the other channels prior to transmission. Alternatively,the transmitted signal may be modified utilizing an inline precode FEXTfilter. This may occur for each transmitter in a multi-channelcommunication system. The coefficients for the FEXT precode filter maybe established by processing that occurs in the transmitter orprocessing that occurs in the receiver.

FIG. 10 illustrates a block diagram of an example embodiment of atransmitter configured with a precode FEXT filter system. This exampleembodiment is configured as a four channel communication device;however, it is contemplated that in other embodiments the principles maybe extended to any communication system having two or more channels.

In reference to the channel A transmitter, a data source not shownprovides one or more signals to a front-end processing module 1004A,which in this embodiment is configured as a mapping module with optionalforward error correction. The output of the front-end processing module1004A feeds into a transmit precode filter 1008A such as would beutilized to account for intersymbol interference. Any type transmitprecode filter 1008A may be utilized. The transmit precode filter may beconsidered analogous to the transmit precode filter as shown in FIG. 4.

The output of the transmit precode filter 1008A connects to a subtractor1012A and one or more precode FEXT filters 1030A_(B)-1030A_(M). Theoutput of precode FEXT filters 1030 generate FEXT cancellation signalsthat are routed to other transmitters at a station so that the generatedcancellation signals may be utilized by the transmitters associated withthe other channels to precode a portion of the FEXT out of thetransmitted signals. For example, precode FEXT filter 1030A_(B)generates a cancellation signal A′_(B) that represents the FEXTcancellation signal, generated by the signal to be transmitted overchannel A, that is provided to the channel B transmitter to precode,i.e., pre-cancel, FEXT coupling that will couple from channel A tochannel B. The precode FEXT filters 1030 may comprise any type of filtercapable of generating a cancellation signal. In one embodiment, thesefilters comprise digital filters. In one embodiment, the precode FEXTfilters 1030 comprise an adaptive digital filter. In one embodiment, theprecode FEXT filters 1030 comprise any type of filter capable ofmanipulating an input signal to generate a FEXT cancellation signal. Theprecode FEXT filters 1030 may comprise, but are not limited to, thefollowing types of filters or any variation thereof: finite impulseresponse filter, infinite impulse response filter or transform domainfilter. It is further contemplated that the precode FEXT filter maycomprise a transposed or transversal configuration, or any otherconfiguration.

As shown in the transmitter associated with channel A, a subtractor1012A receives the output from the transmit precode filter 1008A and thecancellation signal from the precode FEXT filters associated with theother channels, namely filters 1030B_(A), 1030C_(A), 1030M_(A). In oneembodiment, the subtractor 1012A subtracts the cancellation signalsB′_(A), C′_(A) and M′_(A) from the channel A signal, while in anotherembodiment the subtractor 1012A adds the cancellation signals B′_(A),C′_(A) and M′_(A) to the channel A signal.

The output of the subtractor 1012A connects to a digital to analogconverter 1016A, which in turn provides an analog output to a linedriver 1020. The digital to analog converter 1016A and the line driver1020A operate in a manner as would be understood by one of ordinaryskill in the art.

The apparatus associated with the transmitter for channel B, channel C,and channel M operates and comprises generally similar systems andmethods of operation. Accordingly, similar reference numbers to thoseshown for channel A, modified by an appropriate channel identifier, areassociated with each of the other channels' transmit systems.

In one embodiment, the precode FEXT filters 1030 located in the two ormore transmitters operate in conjunction with the ELFEXT filtersdescribed above that are located in the receivers. In such anembodiment, a portion of the FEXT cancellation may occur in thetransmitter and a portion may occur in the receiver. In oneconfiguration, one or more of the ELFEXT filter coefficients of thereceiver FEXT filter are set to zero or another nominal value. It iscontemplated that these coefficients comprise the coefficients thataccount for the non-causal portion of the FEXT coupling as seen by thereceivers. In such an embodiment, it is the precode FEXT filter thatcancels the FEXT that would otherwise be canceled by these coefficients.Hence, the precode FEXT filters 1030 may be considered a non-causalfilter.

Stated another way, to account for the FEXT coupling on the victimsignal that arrives prior to the arrival, at the other receivers, of thedisturber signals that generated the FEXT coupling, certain aspects ofFEXT cancellation are transferred to the precode FEXT filter that may belocated in a transmitter. In one embodiment, the aspects of FEXTcancellation that are transferred to the precode FEXT filter comprisethose aspects that cancel non-casual FEXT. This occurs becausenon-causal FEXT is considered to arrive on the victim channel prior tothe arrival, at other receivers in the multi-channel communicationsystem, of the signals on the disturber channels from which the FEXTcouples. Hence, the duties performed by one or more coefficients of theELFEXT filters may be transferred to the precode FEXT filter 1030. As aresult, certain coefficients of the FEXT filters located in the receivermay optionally be set to zero. In one embodiment, an identical number ofcoefficient values are transferred from the ELFEXT filter to the precodeFEXT filter. Although any number of coefficients values may be set tozero, in one embodiment, 24 receiver filter coefficients are set tozero. In one embodiment, the number of coefficients set to zero is lessthan 12. In another embodiment, the number of coefficients set to zerois between 12 and 24. It is also contemplated that more than 24 of thereceiver filter coefficients may be set to zero.

In an alternative preferred embodiment shown in FIG. 11, the precodeFEXT filters 1030 are located before the transmit precode filter 1008 asshown. This configuration achieves the advantage of simpler trainingsince the standard precode filter is part of the FEXT channel. It iscontemplated that one of ordinary skill in the art may arrive at otherconfigurations that do not depart from scope of the claims that follow.

FIGS. 12A and 12B illustrate an operational flow diagram of an examplemethod of operation of one embodiment of the invention. The method ofoperation shown in FIGS. 12A and 12B encompasses operation of a systemhaving the precode FEXT cancellation and receiver FEXT cancellationcapability. It is contemplated, however, that other embodiments mayimplement only one of these types of FEXT cancellation without departingfrom the scope of the invention. In reference to FIG. 12A, at a step1204, a transmitter at a first station of a multi-channel communicationsystem receives a victim signal from a signal source. It is contemplatedthat the transmitter comprises a victim transmitter because it transmitsa signal onto which other signals will couple. Similarly, for purposesof discussion, the victim signal travels through a victim channel and isreceived by a victim receiver. The term ‘victim signal’ is utilized todistinguish it from a ‘disturber signal,’ which is a signal that causesor is responsible for coupling onto the victim signal. A disturberchannel is defined to mean a channel that carries a signal that couplesonto the victim channel. The disturber signal on the disturber channelis utilized to generate FEXT cancellation signals, which are subtractedfrom the victim signal either before transmission, after reception, orboth. In this embodiment, signals are transmitted from a first stationto a second station.

Next or concurrently, at a step 1206, the other transmitters in themulti-transmitter communication system receive disturber signals thatare to be transmitted from the first station to a second station. Hence,it is contemplated that multiple signals are simultaneously transmittedover two or more channels.

Thereafter, at a step 1208, the victim transmitter processes the victimsignal with one or more precode FEXT filters to generate precode FEXTcancellation signals. The operation and configuration of the precodeFEXT filters is discussed above, and thus is not discussed again. Atstep 1212, the system processes the disturber signals using one or moreprecode FEXT filters to generate FEXT cancellation signals which will beprovided to the victim channel. Thus, in one embodiment, precode FEXTfilters in each transmitter process the outgoing signals, which have notyet been transmitted, to generate a cancellation signal for each of theother channels. In one embodiment, a unique precode FEXT cancellationsignal is generated by each precode FEXT filter for each of the otherchannels, while in another embodiment a single signal is generated byeach transmitter for use by each channel. In one embodiment, processingby the precode FEXT filter comprises generating a signal that has atransfer function generally equivalent to the non-causal portion of theFEXT which will couple onto the other victim channels to which theparticular precode FEXT filter output will be provided. Operation of afilter is generally understood by one of ordinary skill in the art, andhence is not discussed in detail here. At a step 1216, the FEXTcancellation signals from each precode FEXT filter are routed to theappropriate transmitter. Reference to FIGS. 10 and 11 may aid inunderstanding of the routing of FEXT cancellation signals.

At a step 1220, the system subtracts, from the victim signal, the FEXTcancellation signals generated by processing the disturber signals.Similarly, at a step 1224 the system subtracts, from the disturbersignals, the FEXT cancellation signals generated by processing thevictim signals. As a result of the subtracting the FEXT cancellationsignal from each of the signals on the other channels in thetransmitter, the non-causal FEXT coupling is canceled prior totransmission. This provides the advantage of canceling the FEXT couplingthat the FEXT filters located in the receiver would be unable to removebecause the signal generating the coupling will have not yet arrived atthe second station when the cancellation process occurs withinreceivers.

Next, at step 1224, this process is repeated in the other transmitters.In particular, the system subtracts, from the disturber signal, the FEXTcancellation signals generated by processing of the victim signal.Hence, it is contemplated that all the reference transmitters receivesone or more cancellation signals from the other transmitters in themulti-transmitter communication system. In this embodiment, eachtransmitter associated with a disturber channel provides at least onecancellation signal to the other transmitters of the other channels andthe transmitter associated with the victim channel sends at least onecancellation signal to the other transmitters of the other channels. Itis contemplated that in some embodiments, only certain transmitters maygenerate cancellation signals.

At step 1232, the transmitters transmit the victim signal and thedisturber signals from the first station to the second station of themulti-channel communication system. In one embodiment, four channels areutilized to transmit four signal. At step 1236, one or more receiversreceive and process the victim signal to generate a processed victimsignal. The processing may comprise any type of processing that occursin a receiver. In one embodiment, the processing comprises processing toreduce or eliminate intersymbol interference. In general, this type ofprocessing accounts for the effects of the channel. In otherembodiments, resource sharing type processing may occur. As an advantageto the method of operation of this embodiment, the shared processingreduces the computational complexity of burden of the FEXT cancellationfiltering done in the receiver. It is contemplated that the processingof step 1236 may occur prior to or after processing of the signal by anELFEXT filtering.

Turning now to FIG. 12B, at a step 1240, the processed disturber signalis provided to one or more FEXT filters that are located in thedisturber receiver. This may be a part of a more involved processingoperation, as would be understood in light of FIGS. 6 and 8. Also at astep 1244, one or more receivers receive and process the disturbersignals to generate one or more processed disturber signals.

At a step 1244, the receiver system manipulates the processed victimsignal using one or more ELFEXT filters to generate FEXT cancellationsignals. At a step 1246, the receiver systems manipulate the processeddisturber signals using ELFEXT filter(s) to generate FEXT cancellationsignals. At a step 1248, the FEXT cancellation signals generated frommanipulating the disturber signal are distributed to the receiversassociated with the victim channel. Next, at a step 1252, the FEXTcancellation signals generated from the disturber signals are subtractedfrom the victim signal.

A process similar to step 1248 and 1252 occurs at steps 1256 and 1260whereby the receivers of the second station distribute the FEXTcancellation signals from the victim channel to receivers associatedwith the disturber channel. Thereafter, at step 1260, these signals aresubtracted from the disturber signals. It is contemplated that thismethod of operation may occur continuously to cancel FEXT couplingduring data transmission between the first station and the secondstation. At a step 1264, the operation continues in this manner togenerate and output a signal having the FEXT coupling cancelled. It iscontemplated that each signal can be labeled a disturber signal inrelation to one or more of the other signals on the other channels.Hence, each channel suffers from coupling from the other channels, andhence it follows that both channels may be considered a disturberchannel and a victim channel, depending on the point of reference.

FIG. 13 illustrates a more detailed operational flow diagram of anexample method of operation of the system of FIG. 6. This is but onepossible example method of operation and, as such, it is contemplatedthat other methods, such as those offered in the other figures, may beenabled without departing from the claims that follow. In addition, thisexample method of operation only discusses removal of coupling from avictim signal, but it is contemplated that this FEXT coupling removalprocess may occur within any or all of the receivers in a multi-channelcommunication system. At a step 1304, the receiver receives a victimsignal. It is assumed that the victim signal contains unwanted couplingfrom a disturber channel. Thereafter, at step 1308, the disturber signalis received. The disturber signal is utilized to generate a cancellationsignal that when combined with the victim signal, will reduce oreliminate the coupling from the disturber signal to the victim signal.

Accordingly, at a step 1312, the receiver associated with the victimchannel utilizes feed-forward and/or feedback filtering to generate afirst processed victim signal. Similarly, at step 1316, the receiverassociated with the disturber signal utilizes a feed-forward and/or afeedback filter to generate a first processed disturber signal.Processing of the disturber signal with the feed-forward and/or feedbackfilters aids in signal processing burden when generating thecancellation signal, discussed below, in that the feed-forward and/orfeedback filtering accounts for the effects of the channel on thecoupling signal while the ELFEXT filter may be trained to account forthe coupling and not the effects of the channel.

Thereafter, at a step 1320, the first processed disturber signal isprovided to an ELFEXT filter configured to generate, at a step 1324, acancellation signal that will cancel the coupling from the disturbersignal onto the victim signal. The configuration of the ELFEXT filter isdescribed above, and hence a duplicate discussion is not repeated. Next,at a step 1328, the FEXT cancellation signal from the ELFEXT filter issubtracted from the first processed signal to create a second processedsignal. In one embodiment, the second processed signal comprises theoutput of a feedback filter with the cancellation signal subtractedtherefrom. At a step 1332, the operation subtracts the second processedsignal from the incoming signal, such as after processing by afeed-forward signal, to remove FEXT coupling from the received victimsignal. In other embodiments, the second processed signal may begenerated in other manners or may be subtracted from the incoming signalat a different stage of processing.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention.

1. A multi-channel communication system having a first station and asecond station configured to communicate over two or more channelscomprising: the first station having two or more transmitters configuredto send two or more transmitted signals over two or more channels fromthe first station to the second station; the second station having twoor more receivers configured to process a received signal, wherein eachreceived signal comprises the transmitted signal and one or morecoupling signals and wherein at least one receiver comprises: a decisiondevice configured to generate a decision output based on at least thereceived signal and a modified decision output; a feedback systemconfigured to generate the modified decision output and combine themodified decision output, the received signal, and one or more incomingFEXT cancellation signals; an adder configured to add the modifieddecision output from the decision output to create an intermediatesignal; and one or more ELFEXT filters configured to process theintermediate signal to create one or more outgoing FEXT cancellationsignals.
 2. The system of claim 1, wherein the communication system isfurther configured to transmit data from the second station to the firststation.
 3. The system of claim 1, wherein the one or more incomingcancellation signals comprise one or more cancellation signalsconfigured to remove FEXT coupling from the received signal.
 4. Thesystem of claim 1, wherein the adder is further configured to add themodified decision output and one or more incoming cancellation signalsfrom the decision output to create the intermediate signal.
 5. Thesystem of claim 1, wherein the ELFEXT filter is configured to accountfor an ELFEXT portion of FEXT coupling.
 6. The system of claim 1,wherein each receiver at the second station generates a uniquecancellation signal tailored for each of the other receivers at thesecond station.
 7. The system of claim 1, wherein the feedback systemcomprises a decision feedback filter.
 8. The system of claim 1, whereinthe decision device comprises a slicer.
 9. The system of claim 1,wherein the multi-channel communication system comprises a four channelcommunication system configured to operate in accordance with anEthernet Communication Standard.
 10. The system of claim 1, wherein thetwo or more transmitters of the first station further comprise two ormore FEXT precode filters configured to modify the two or more signalsprior to transmission to cancel FEXT coupling.
 11. The system of claim10, wherein each transmitter comprises one or more FEXT precode filtersconfigured to generate and provide one or more precode cancellationsignals to other transmitters.
 12. A multi-channel communication systemconfigured to reduce noise comprising: one or more transmittersconfigured to transmit a first signal on a first channel and a secondsignal on a second channel; and a first receiver configured to receive athird signal on the first channel and a second receiver configured toreceiver a fourth signal on the second channel, wherein the third signalcomprises the first signal and a first interference component and thefourth signal comprises the second signal and a second interferencecomponent wherein; the first receiver comprises: a first feedback filterloop configured to receive the third signal and reduce interference onthe third signal, the output of the first feedback filter loopcomprising a first feedback filter loop output; a first decision devicehaving a first decision device output configured as part of the firstfeedback filter loop; a first device configured to receive a secondcancellation signal from the second receiver and combine the secondcancellation signal with the first feedback filter loop output; a firstfilter configured to receive at least the decision device output andgenerate a first cancellation signal; and the second receiver comprises:a second feedback filter loop configured to receive the fourth signaland reduce interference on the fourth signal, the output of the secondfeedback filter loop comprising a second feedback filter loop output; asecond decision device having a second decision device output configuredas part of the second feedback filter loop; a second device configuredto receive the first cancellation signal from the first receiver andcombine the first cancellation signal with the first feedback filterloop output; and a second filter configured to receive at least thesecond decision device output and generate the second cancellationsignal.
 13. The system of claim 12, wherein the first device and thesecond comprise summing junctions.
 14. The system of claim 12, whereinthe first feedback filter loop and the second feedback filter loop bothcomprise a decision feedback filter configured to reduce intersymbolinterference.
 15. The system of claim 12, wherein the multi-channelcommunication system has four channels and the interference comprisesFEXT coupling.
 16. The system of claim 12, wherein the first filter andthe second filter comprise digital filters having coefficient valuesselected to generate cancellation signals that cancel FEXT coupling. 17.The system of claim 12, wherein the one or more transmitters furthercomprise precode FEXT filters, wherein each precode FEXT filter isconfigured to generate a cancellation signal that can be combined with asignal, prior to transmission of the signal, to pre-cancel FEXTcoupling.
 18. The system of claim 12, wherein at least one of the one ormore transmitters is configured to generate an outgoing precodecancellation signal and receive an incoming precode cancellation signalfrom another transmitter.
 19. A receiver for use in a multi-channelcommunication system to cancel coupling on a transmitted signal andreduce intersymbol interference, wherein a distorted version of thetransmitted signal and FEXT coupling comprise a combined signal, thereceiver comprising: a first device configured to receive and combine afeedback signal with the combined signal to create a decision deviceinput signal; a decision device configured to process the decisiondevice input signal to generate a discrete output; a decision feedbackequalizer configured to receive and process the discrete output togenerate an equalizer output; a second device configured to combine anincoming cancellation signal with the equalizer output to create thefeedback signal; and one or more ELFEXT filters, each configured togenerate an outgoing cancellation signal that is related to the discreteoutput, wherein the outgoing cancellation signal is tailored to cancelFEXT coupling on another channels in the multi-channel communicationdevice.
 20. The system of claim 19, further comprising a third deviceconfigured to combine the discrete output and the one or more delayedcancellation signals to create an input to the decision feedbackequalizer.
 21. The receiver of claim 19, wherein the decision devicecomprises a slicer having ten output levels.
 22. The receiver of claim19, wherein each multi-channel communication system comprises a stationand each station comprises four receivers.
 23. The receiver of claim 19,further comprising a feed forward filter configured to process thecombined signal to reduce intersymbol interference on the combinedsignal.
 24. A receiver in a multi-receiver system configured to receivetwo or more signals via two or more channels, each respective receivercomprising: an input configured to accept a received signal; a decisiondevice configured to quantize a decision device input signal to one oftwo or more decision values, wherein the decision device input signal isbased on the received signal; a first filter configured to process thedecision values to create a first filtered signal; one or more secondfilters configured to process the decision values and the first filteredsignal to create an outgoing cancellation signal tailored to cancelcoupling on one or more other channels; one or more devices configuredto: receive one or more incoming cancellation signals from otherreceivers in the multi-receiver system; and process the one or moreincoming cancellation signals, the first filtered signal, and thereceived signal to cancel unwanted coupling in the received signal. 25.The receiver of claim 24, wherein the first filter comprises a digitalfilter configured to generate a feedback signal that reduces intersymbolinterference.
 26. The receiver of claim 24, wherein the decision devicequantizes the decision device input signal to any one of ten valuesbased on a comparison to predetermined thresholds.
 27. The receiver ofclaim 24, wherein the one or more second filters comprise digitalfilters having two or more coefficients and the one or more secondfilters and the first filter are configured to cancel coupling andreduce intersymbol interference.
 28. The receiver of claim 24, furthercomprising a third filter comprising a feed forward filter that isconfigured to process the received signal to reduce intersymbolinterference.
 29. The receiver of claim 24, wherein the one or moredevices comprise one or more subtractors.
 30. A method for reducinginterference in a multi-channel communication system having two or morereceivers and two or more channels, the method of the first receivercomprising: receiving a first signal on a first channel with a firstreceiver and a second signal on a second channel with a second receiver;combining a feedback signal with the first received signal to create afirst combined signal; processing the first combined signal to reduceintersymbol interference in the first combined signal to create aprocessed signal, the interference created by passage of the firstsignal through the first channel; combining the processed signal with atleast a first cancellation signal received from at least the secondreceiver to create a feedback signal; combining the feedback signal withthe first combined signal to create a second combined signal; andprocessing the second combined signal to generate at least a secondcancellation signal.
 31. The method of claim 30, wherein the combining afeedback signal with the first received signal cancels FEXT coupling inthe first received signal.
 32. The method of claim 30, whereinprocessing the first combined signal comprises performing decisionfeedback equalization on the signal to generate a signal that reducesintersymbol interference.
 33. The method of claim 30, wherein processingthe second combined signal to generate at least a second cancellationsignal comprises filtering the second combined to isolate ELFEXTcoupling.
 34. The method of claim 30, wherein the second receiver isconfigured similarly to the first receiver and the second receivergenerates the first cancellation signal and receives the secondcancellation signal from the first receiver.
 35. The method of claim 30,further comprising delaying the first cancellation signal to achieveproper timing.
 36. A receiver for FEXT cancellation in a multi-channelcommunication system comprising: a feedback loop comprising: a firstdevice configured to combine a received signal with a feedback signaland one or more incoming cancellation signals to create a combinedsignal; a decision device configured to process the combined signal togenerate a decision output; a first filter configured to generate thefeedback signal based on the decision output and the one or moreincoming cancellation signals or a delayed version of the one or moreincoming cancellation signals, wherein the one or more incomingcancellation signals are received from one or more other receivers inthe multi-channel communication system; and one or more second filtersconfigured to receive at least the decision output and generate one ormore outgoing cancellation signals which are routed to other receiversin the multi-channel communication system.
 37. The receiver of claim 36,wherein the first device comprises a subtractor or summing junction, thefirst type filter is configured to account for the effects of thechannel and the one or more second type filters are configured toaccount for coupling.
 38. The receiver of claim 36, wherein FEXTcancellation is performed by the first filter and an incomingcancellation signal.
 39. The receiver of claim 37, further comprising afeed-forward filter configured to process the received signal prior tothe received signal arriving at the feedback loop.
 40. The receiver ofclaim 37, wherein a receiver is associated with each channel in a fourchannel communication system and each receiver receives an incomingcancellation signal from each of the other receivers.
 41. A method forcanceling coupling in a multi-channel communication system having two ormore receivers comprising: receiving a signal over a channel; receivingat least one cancellation signal from at least one of the otherreceivers in the multi-channel communication system; processing thesignal to account for the effect of the signal passing through thechannel thereby generating a processed signal; combining the processedsignal and the one or more cancellation signals from the other receiversto generate a feedback signal; combining the feedback signal with thereceived signal to cancel coupling in the received signal; andgenerating one or more outgoing cancellation signals as a result ofprocessing the processed signal and providing at least one outgoingcancellation signals to at least one of the other receivers in themulti-channel communication system.
 42. The method of claim 41, whereingenerating one or more outgoing cancellation signals comprisesgenerating one or more cancellation signals with a filter configured toisolate the ELFEXT coupling.
 43. The method of claim 41, furthercomprising combining the feedback signal with the one or morecancellation signals from the other receivers prior to combining thefeedback signal with the received signal to reduce noise in the receivedsignal.
 44. The method of claim 41, wherein processing comprisesprocessing with a decision feedback equalizer.
 45. The method of claim44, wherein processing further comprises quantizing the combination ofthe received signal and the one or more cancellation signals to one ofone or more discrete levels prior to processing.
 46. A system forcanceling one or more FEXT signals that have coupled onto a transmittedsignal to create a modified signal in a multi-channel communicationdevice comprising: means for receiving the modified signal over achannel in the multi-channel communication system; means for combiningthe modified signal with a feedback signal to isolate the transmittedsignal; means for generating the feedback signal comprising: means forfiltering the isolated transmitted signal to create a filtered signal;means for receiving and combining the filtered signal and one or morecancellation signals received from other receivers in the multi-channelcommunication device; and means for generating one or more cancellationsignals to be outputted to one or more other receivers.
 47. The systemof claim 46, wherein means for generating comprises a decision deviceand a filter.
 48. The system of claim 46, wherein the means forgenerating one or more cancellation signals comprises: means forcombining the transmitted signal and the feedback signal to create acancellation filter input signal, and means for processing thecancellation filter input signal to create cancellation signals to beprovided to other receivers.